APPLICATION OF THE ANALYTICAL ELECTRON MICROSCOPE TO THE STUDY OF GRAIN BOUNDARY CHEMISTRY

High spatial resolution X-ray spectroscopy in the analytical electron microscope (AEM) is a powerful tool for the study of changes in chemistry which occur at grain boundaries in metals and ceramics. Two major advantages are realized through the use of the AEM in these studies : the ability to obtain accurate quantitative microchemical analysis of grain boundary regions, and the capability for determining the structural and crystallographic characteristics of the boundaries on which the chemical measurements were made. In this presentation, experimental procedures for AEM microanalysis are briefly discussed, with emphasis on the capabilities and limitations of the technique. The application of these procedures is illustrated using two important cases which serve to demonstrate a wide range of possible behavior : the equilibrium segregation of Fe to grain boundaries in MgO due to space-charge considerations, and the depletion of Cr at grain boundaries in stainless steel caused by the precipitation of Cr-rich carbides. A mathematical model will be utilized to extract the maximum information from the experimental data in each case. The effect of instrumental and structural parameters on the composition profiles in both systems will be shown. The types of information provided by the AEM, and its relation to theories of the development of chemistry variations at grain boundaries, will be discussed in detail.